The present invention relates particularly to an expandable framework structure which is light, highly rigid and easy to stow away for use for a large-diameter expandable antenna designed to be mounted on an artificial satellite.
Recently, the performance of spacecrafts such as the Space Shuttle and Ariane rocket has been improved, and the reliability of these spacecrafts has also been enhanced. Because of this, the economic merits in using the spacecrafts in space have been acknowledged.
A large-sized expandable antenna is inevitably necessary for establishing communications between moving objects such as seacrafts, vehicles and so forth; and a high level of competition in the development of expandable framework structure schemes for use in constituting such a large-sized expandable antenna has been taking place. There is also a plan to build gigantic space bases in space. In addition to its application in spacecraft, the expandable framework structure (expandable truss structure) as a basic structure scheme for such gigantic space bases is now one of the crucial development themes to be dealt with in order to realize the construction of such space bases. It is widely accepted that the expandable structure scheme is one of the most economical ways of constructing a gigantic structure with relative ease.
FIGS. 9 and 10 are, respectively, front and side views of an expandable framework structure, which is in a stowed state, used for a conventional expandable antenna such as the one disclosed in the official gazette of Japanese Patent Public Disclosure No. 59-28704 (1984), and FIGS. 11 and 12 are also front and side views of the same which is in an unfolded state, respectively. In these figures, reference numerals 11a, 11b denote hinges, and reference numerals 12a, 12b frame members adapted to be connected to each other by means of the hinges 11a, 11b at the ends thereof so as to form a hoop-like shape as a whole. Reference numeral 13 denotes support wires, and reference numeral 14 a mesh-like flexible film adapted to function as the reflector of the antenna. This flexible film 14 is designed to be expanded and retained in place by means of the support wires 13, as shown in FIG. 11.
The operation of the above-described expandable framework structure shown in FIGS. 9 to 12 as used with the conventional expandable antenna will be described next. When an artificial satellite is launched, the frame members 12a, 12b which are connected to each other by the hinges 11a, 11b at the ends thereof are, as shown in FIG. 9, folded at the hinged portions and are fixed to the side wall of the satellite by means of fixing devices (not shown). When the satellite has been sent into orbit, the fixing devices are released, and the frame members 12a, 12b are then developed by the action of torque (driving force) generated by spiral springs (not shown) incorporated in the hinges 11a, 11b and are looked in place by means of incorporated latch devices (not shown) when they are developed to a predetermined position, whereby a hoop-shaped frame structure is formed as a whole, as shown in FIG. 11. At the same time as the frame members 12a, 12b are developed, the flexible film 14 secured to the frame members by means of the support wires 13 are also expanded, and when the frame members 12a, 12b have formed the hoop-shaped frame structure, the flexible film 14 is caused to spread out inside the frame members 12a, 12b, and thereby forms an electric wave reflector for the expandable antenna.
In addition, FIG. 13 shows the structure of another conventional expandable framework structure disclosed in the "IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION", Vol. AP-17, 4th issue (1969). In the figure, reference numeral 21 denotes foldable members constituting the upper and lower triangular frameworks for the expandable framework structure and adapted to be folded at the intermediate positions thereof, and reference numeral 22 diagonal members for supporting the upper and lower triangular frameworks, and reference numeral 23 denotes joints for pins connecting the foldable members 21 and the diagonal members 22.
FIG. 14 is a side view, partially cut away, showing the constitution of the foldable member used with the expandable framework structure shown in FIG. 13, wherein the detailed constitution of the intermediate portion of the foldable member 21 where folding of the same actually takes place is shown. In the figure, reference numeral 21 denotes foldable members; reference numerals 25a, 25b denote rotatable hinge levers comprising two sheets which are pin connected to each other, reference numeral 26 a spiral spring mounted on one 25b of the hinge levers at the joint portion thereof to rotate the hinge levers 25a, 25b in the direction in which the foldable member 21 is developed, and reference numerals 27a, 27b denote connecting pins for connecting the hinge levers 25a, 25b to the foldabIe member 21 with reference numeral 27cdenoting a connecting pin for connecting the foldable members 21 to each other at a position making an intermediate portion of the so connected foldable members 21. FIG. 15 is a side view showing the configuration of the foldable member 21 shown in FIG. 14 which is in an expanded state. FIG. 16 shows the configuration of the expandable framework structure shown in FIG. 13 which is being expanded. The foldable members 21, diagonal members 22 and joints 23 are also shown as being expanded.
The operation of the conventional expandable framework structure shown in FIGS. 13 to 16 will be described below. Initially the expandable framework structure is stowed and retained in that state by means of retaining cables (not shown). Once a command for expansion is sent from the ground, the retaining cables are cut and broken by virtue of an explosion of an explosive tube(s), whereby the expandable framework structure is put ready to expand and starts to expand by the action of the elastic force of the spiral springs 26. The spiral springs 26 rotate associated hinge levers 25a, 25b by virtue of the elastic force thereof, and this in turn rotates the foldable members 21 about associtated connecting pins 27c, whereby the foldable members 21 are unfolded. The joints 23 on the upper and lower sides are caused to radially spread out as the foldabIe members 21 are caused to unfold, and the expansion of the expandable framework structure thus progresses. When the foldable members 21 have been linearly unfolded, the rotational torque generated by means of the hinge levers 25a, 25b which are actuated by the action of the elastic force of the spiral spring 26, balances with the pressure applied to the plane of contact between the respective connected ends of the foldable members 21. This serves to cease the expansion of the foldable members 21, so as to allow them to form a linear configuration as shown in FIG. 15. In this state, the hinge levers 25a, 25b are substantially in parallel to each other, and the angle formed between the center axis of the so expanded foldabIe members 21 and the hinge levers 25a, 25b is as low as about 15 degrees. Due to this configuration, even if a force is applied to the foldable members 21 in an attempt to re-fold the same, the hinge levers 25a, 25b are caused to thrust out at each other in the axial direction. The hinge levers 25a, 25b thus function as a latch for preventing the re-folding of the foldable members 21.
In the expandable framework structure shown in FIGS. 9 to 12 that is constructed as described above and which was used with a conventional expandable antenna, in a case where, for instance, it is used in constructing a large-diameter expandable antenna, the frame members 12a, 12b which are connected to each other at the ends thereof by means of the hinges 11a, 11b are first expanded to form a single large-diameter hoop-shaped frame structure, and the mesh-like flexible film 14 is supported only by the structure. The flexible film 14 has no highly rigid members thereon except the peripheral portion. This reduces the overall rigidity of the antenna, and hence often causes a problem that the antenna fails to maintain its desired shape in the event that external force acts on the antenna in an attempt to control the direction thereof.
Furthermore, in the case of the conventional expandable framework structure shown in FIGS. 13 to 16 that is constructed as described above, attachments like the hinge levers 25a, 25b which are essentially unnecessary for the construction of the antenna must be provided. This serves to increase the number of components used and hence causes problems such as those of reduction in reliability, weight increase, increase of volume when stowed and so forth.